This is a U.S. national stage of application No. PCT/DE98/01544, filed on Jun. 03, 1998. Priority is claimed on that application and on the following application:
Country: Germany, Application No.: 197 24 232.4, Filed: Jun. 03, 1997.
1. Field of the Invention
The invention relates to a process for producing slabs having a thickness D greater than 100 mm, at casting speeds v less than 3 m/min, in a continuous casting installation in which melt is supplied to a permanent mold from a storage reservoir via an immersion nozzle and from which, on the aperture side, the strand shell enclosing a liquid crater is withdrawn into a strand guidance frame More particularly, the invention relates to; a bow-type continuous casting installation, and to a suitable appliance for out the process.
2. Discussion of the Prior Art
Steel Research 66 (1995) No. 7, pp. 287 to 293 xe2x80x9cFlow dynamics in thin slab caster moldsxe2x80x9d has discloses a test layout in which an immersion nozzle which is attached to a tundish projects into a permanent mold. The permanent mold used here, with a thickness of approximately 80 mm, is the typical size for an installation for producing thin slabs and, during use of an immersion nozzle (FIG. 10 of the Steel Research reference) which has an open aperture, exhibits a central jet which projects deep into the liquid crater of the slab.
In a further configuration (FIG. 4 of the Steel Research reference), a baffle element, which diverts the liquid melt toward two openings on the narrow sides of the immersion nozzle, is provided at the aperture of the immersion nozzle. FIG. 5 of the reference shows that two partial streams are produced, which together with high energy of each individual flow filament, lead to turbulence in the melt.
German reference DE 43 20 723 has discloses a submerged nozzle, in particular for casting thin slabs, which has a lower section comprising side walls which are guided parallel to one another. Before entry to the lower section, a transverse web is provided, which diverts the melt flow in the direction of the widening of the lower flow shaft. The narrow sides of this immersion nozzle, which is provided in particular for thin slab installations, are guided parallel to one another.
The immersion nozzles which are known from the publications mentioned above produce a casting jet which, at a relatively high speed, penetrates into the liquid crater to corresponding depths.
In view of the above-mentioned prior art, the object of the present invention is to provide a process and a corresponding appliance for producing slabs, in which concentrations of impurities are avoided and in particular acid-gas-resistant steel goods can be cast even in bow-type continuous casting installations.
According to the invention, the liquid melt which is supplied to the permanent mold enters the liquid crater of the slab over a wide front and at speeds which are higher than the strand withdrawal speed. With regard to the cross section, the melt supplied has a rectangular profile and even at a depth of no more than 2 m in the liquid crater has already reached the same speed as the slab.
The speed vK of the melt supplied, which enters the permanent mold; has the following relationship with respect to the strand withdrawal speed vB:
vK:vB=6:1 to 60:1.
In an advantageous embodiment of the invention, the liquid melt supplied is guided into the liquid crater with an entry profile which is formed as a rectangle, the clear width d of the rectangle having the following relationship with respect to the narrow side of the permanent mold D:
d:D=1:3 to 1:40
and the breadth b of the rectangle has the following relationship with respect to the wide side of the permanent mold B:
b:B=1:7 to 1:1.2.
The flow filaments leaving the immersion nozzle flow into the liquid crater at a width angle of xcex1=15 to 30xc2x0 with respect to the slab withdrawal direction. With regard to the side D of the permanent mold narrow side, the liquid melt supplied impinges on the liquid crater over a depth T=0.1 to 1.5xc3x97D. The immersion nozzle used for this purpose has narrow side walls which, with regard to the center axis, open out conically at an angle xcex1 of 15 to 30xc2x0. The free cross section a of the aperture of the casting section of the immersion nozzle has the following relationship with respect to the internal cross section A of the permanent mold:
a:A=1:30 to 1:300.
In this case, the clear width d of the casting section of the immersion nozzle has the following relationship with respect to the narrow side D of the permanent mold:
d:D=1:2 to 1:40.
The profile produced in the permanent mold by means of the proposed process moreover has a positive effect on the movement of the melt in the region of the melt level in the permanent mold and on its behavior with regard to the mold powder.
When casting according to the invention, it was surprisingly established that the known differences in concentration over the cross section of the slab did not occur and that the degree of purity, based on nonmetallic inclusions, was significantly improved.
The proposed process makes it possible to produce slabs for steel goods where there are high demands both with regard to the nonmetallic degree of purity and also with regard to the freedom from segregation, as required, for example, for acid-gas-resistant steel goods.
Furthermore, when casting according to the invention, the reduced speed at which the steel flows into the liquid crater situated in the strand shell reduces the total solidification time. As a result, it is possible, on the one hand, to increase the specific casting capacity of the installations, or, on the other hand, to reduce the specific secondary cooling with a view to improved surface quality.